Uncovering the phase diagram of QCD matter is one of the major goals of heavy-ion collision research, and the founding reason behind the ongoing Beam Energy Scan (BES) program at the BNL Relativistic Heavy Ion Collider (RHIC). Current understanding of the evolution that QCD matter undergoes at extreme conditions is facilitated by numerous experimental and theoretical advancements to date. The importance of quark and gluon degrees of freedom for the dynamics of very high-energy collisions is strongly supported by comparisons of experiment to theoretical models [1]}, [2]}, and suggests that the quark-gluon plasma (QGP) is produced in these events. Collective behavior of matter created in such collisions has been measured [3]} and reproduced in hydrodynamics simulations [4]}, [5]}, indicating that for a considerable fraction of a heavy-ion collision's evolution, it can be thought of as a thermal system described by an equation of state (EOS). The exact nature of the transition between the QGP and a hadron gas is studied within a number of approaches. At finite temperature and negligible baryon chemical potential, first-principle calculations in lattice QCD (LQCD) predict a transition of the crossover type [6]}. This result has been further supported with a Bayesian inference approach [7]}, where the range of equations of state most consistent with experimental data at high energies has been identified and shown to include the LQCD EOS. On the other hand, numerous chiral effective field theory models predict that at finite baryon number density the transition between hadronic and quark-gluon matter is of the first order [8]}. If this is the case, the phase diagram of QCD matter contains a QGP-hadron coexistence line, ending in a critical point.
